1. Field of the Invention
The present invention relates to a transducer for coupling an electromagnetic signal between a coaxial cable and a waveguide.
2. Description of the Related Art
Coaxial cables and waveguides are used extensively for transmission of electromagnetic signals at microwave and other frequencies, and are suitable for different types of applications. It is often necessary to couple an electromagnetic signal from a coaxial cable to a waveguide or vice-versa. A coupler or transducer is required to perform this transition with minimum signal loss and maximum bandwidth.
Most coaxial-to-waveguide couplers can be classified into three general types, as described in an article entitled "Design of Simple Broad-Band Wave-Guide-to-Coaxial-Line Junctions", by S. Cohn, in Proceedings of the I. R. E., Sep. 1947, pp. 920-926. In the first type, the inner and outer conductors of the coaxial line contact opposite respective walls of the waveguide. In the second type, the inner conductor projects as a probe only part way into the waveguide. In the third type, the inner conductor connects to a coupling loop inside the waveguide.
The second type of coupler, to which the present invention most closely relates, is described in greater detail in an article entitled "IDEAL W. G. TO COAX TRANSITIONS USING A F. B. M. MONOPOLE", by F. De Ronde, in 1988 IEEE MTT-S Digest, pp. 591-594. With reference being made to present FIGS. 1 and 2, a waveguide 10 includes an elongated hollow tubular wall 12 having a rectangular cross-section. The tubular wall 12 includes an upper wall 12a, a lower wall 12b, and side walls 12c and 12d which are joined together at their adjacent edges. The wall 12 may be formed as a single piece by metal extrusion or other suitable process. Alternatively, the walls 12a, 12b, 12c and 12d may be fabricated separately and joined together by welding or the like.
The lower wall 12d of the tubular wall 12 is formed with a hole 12e. An end portion 14a of a center conductor 14 of a coaxial cable 16 protrudes into the waveguide 10 through the hole 12e. The cable 16 is joined to the waveguide 10 by a conventional connector which is not shown in the drawing.
The end portion 14a of the center conductor 14 acts as a transducer probe. An electromagnetic signal propagating through the coaxial cable 16 is electromagnetically induced into the waveguide 10 through coupling between the end portion 14a and the waveguide 10. Conversely, an electromagnetic signal propagating through the waveguide 10 is electromagnetically induced into the coaxial cable 16 through the end portion 14a.
The end portion 14a protrudes into the waveguide 10 adjacent to an end wall 12f of the tubular wall 12 which constitutes a short. In order to match the coaxial cable 16 to the waveguide 10 with minimum signal loss and maximum bandwidth, the geometry of the transition must be designed precisely.
As illustrated in the drawing, the main dimensions which affect the coupling between the coaxial cable 16 and waveguide 10 are the height H and width W of the inner cross-section of the tubular wall 12, the distance L between the center of the end portion 14a and the end wall 12f, the distance h by which the end portion 14a protrudes into the interior of the waveguide 10 above the inner surface of the lower wall 12d, the diameter 2a of the end portion 14a, and the diameter 2b of the hole 12e.
It is also possible to adjust the coupling by offsetting the end portion 14a right or left of the center position as viewed in FIG. 2, although this results in increased signal loss. Other expedients for adjusting the coupling as described in the article to De Ronde include providing shunt or series capacitance stubs in the waveguide 10 adjacent to the end portion 14a.
The distance L is generally on the order of 1/4 wavelength at the desired operating frequency, and has a major effect on the bandwidth of the transition. However, the optimal distance L is a function of numerous complex variables and is generally determined empirically. The end wall 12f is shown as being constituted by the inner end of a plunger or piston 18 which slidably fits inside the tubular wall 12 and facilitates fine tuning of the assembly by adjusting the distance L. Some of the factors which affect the optimal distance L and a simplified design procedure are described in an article entitled "The Optimum Piston Position for Wide-Band Coaxial-to-Waveguide Transducers", by W, Mumford, in Proceedings of the I. R. E, Feb. 1953, pp. 256-261.